Version 1.5 works only in Firefox 3. You can download older versions, which work in Thunderbird, Sunbird and older versions of Firefox, after clicking on "See All Versions".

The theme works in Firefox, Thunderbird and Sunbird. (See also: http://gulp21.uplink47.net/addons/History.html)

In order to modify the look of MozXP use userChrome.css Codes or Gulp21 Theme Modifier (for version 1.5.2 and later, https://addons.mozilla.org/firefox/addon/7514). you can free download MozXP 1.5.2 now.